Spectral measurements of objects are required by various industries for different applications. The term ‘object’ herein refers to continuous printed substrates or discrete objects such as packages, fruits, vegetables, plastic toys, boxes and the like. For example, in the food industry, spectral measurements are used for sorting and quality control. In the biological materials industry, spectral measurements are used for the examination of dye markers absorbed by various cells or organisms, or to non-invasively identify pigments in a single cell or clusters of cells and to map the spatial organization of phototrophic groups in complex microbial communities. In the chemical industry, spectral measurements are used, for example, to identify the presence of pollutants in the air, by distinguishing the pollutants from the nitrogen, oxygen, water and other expected constituents.
In particular, spectral measurements are used in the printing industry for determining chromatic characteristics of objects. The chromatic characteristics may be represented, for example, by the estimated coordinates of a printed color in a color space (e.g., CIEXYZ, CIELUV, CIELAB, RGB, CYMK, HIS and the like), determined according to measurements of the light, which is either reflected from or transmitted through the object. The chromatic characteristics may further be represented by the optical reflection densities of the inks deposited on a printed substrate or an object. The chromatic characteristics may also be represented by the spectral composition of the light reflected from the printed substrate.
Objects, which at least a portion thereof, transmit at least a portion of incident light thereon, are herein referred to collectively as non-opaque objects. The term ‘non-opaque’ is defined, for example, in accordance with the ISO 13655 standard, where “opaque” is defined as a material with opacity greater than 99. A material behind the non-opaque objects may affect the reflectance factor of light impinging on the non-opaque objects. Alternatively, the material behind the non-opaque objects may affect the transmittance factor of light illuminating the non-opaque objects. When spectral measurements are made, the material behind the non-opaque object is referred to herein as a ‘backing.’ The reflectance factor can be defined as the ratio of light reflected from the printed substrate to light reflected from a perfect reflecting diffuser, both illuminated with the same light from the same direction. Similarly, the transmittance factor can be defined as the ratio of light transmitted through the object to light transmitted through a perfect transmitting diffuser, both illuminated with the same light from the same direction. The reflectance or transmittance factors may in turn affect the estimation of chromatic characteristics of the non-opaque objects, which are determined according thereto. For example, the reflectance factor is used when determining the X, Y and Z tristimulus values (e.g., according to the CIE 1931 XYZ color space) according to spectral measurements of the reflected light from the non-opaque object. Thus, for example, the determined X, Y and Z tristimulus values of a green color printed on a non-opaque material with a shiny aluminum backing will be different than the determined X, Y and Z tristimulus values of the same green color printed on the same non-opaque material with a white material backing.
To achieve repeatability and reproducibility of measurements of chromatic characteristics from a non-opaque object, as well as to provide a basis for comparing measurements between different measurement systems, a standard backing may be placed behind the non-opaque object during spectral measurements. During such spectral measurements, the printed material may need to be in proximate contact with the backing. The term ‘repeatability of measurement’ relates herein to a measurement of a sample, which can be repeated (i.e., for that same sample), with the same measuring instrument, under substantially the same measurement conditions, with substantially the same results (i.e., within a determined tolerance). The ‘reproducibility of measurement’ relates herein to a measurement of a sample, performed by an observer, with a measurement instrument, under specified measurement conditions, which can be reproduced by another observer, with substantially the same measurement instrument, under substantially the same measurement conditions, with substantially the same results (i.e., within a determined tolerance). For example, according to the ISO 13655 standard, the backing may be black, referred to as a black backing, or white, referred to as a white backing. Specifically, Annex A of the ISO 13655 standard defines the requirements of the white and black backings. The ISO 13655 standard further recommends that the measurements of light reflected off the non-opaque object, placed on and in contact with the backing, should be corrected relative to the reflection measurements of the backing alone. Similarly, other standards and industry accepted specifications such as those provided by the American Society for Testing and Materials (ASTM), the American National Standards Institute (ANSI), the Deutsches Institut für Normung (DIN), Fogra, and the Flexographic Technical Association (FTA), have various existing and evolving standards and specifications for colorimetric measurements. Such standards and specifications are, for example, FIRST, SNAP, Specifications Web Offset Publications (SWOP), General Requirements for Applications in Commercial Offset Lithography (GRACOL) and the like.
Systems and methods for the measurement of chromatic characteristics of non-opaque objects are known in the art. Also known in the art are methods and systems for stabilizing webs and measuring the thickness of webs. The publication “Short Communication. Automatic Inspection of the Pomegranate (Punica granatum L.) Arils Quality by Means of Computer Vision” by Blasco et al., is directed to a system for ensuring the quality and homogeneity of the color of commercial batches of pomegranate arils. The arils are singulated and transported over a conveyor belt. Images of the arils are acquired using two cameras to distinguish between arils and raw material by estimating the color of the objects and classifying the arils by size and color. The arils are separated into categories by means of air ejectors. The color of the conveying belts is selected to maximize the contrast, in the acquired image, between the arils and the background.
U.S. Pat. No. 5,056,431 to Sainio, entitled “Bernoulli-effect web stabilizer” is directed to a system and method for stabilizing a moving web within a printing press having one or more printing units. The system includes an air driven stabilizer structure disposed proximate to the printing units. The system stabilizes a printed web as it leaves a final print cylinder using a Bernoulli-effect stabilizer which is positioned proximate to the point where the web leaves the final print cylinder. A scanner, mounted in the vicinity of the stabilizer, accurately detects a desired printed image within a narrowly circumscribed depth-of-field. An air bar may be disposed immediately downstream of a print cylinder to reduce variations in the arc length of blanket follow. The air bar may also be disposed proximate to the upper surface of the web, where the surface tends to adhere to the print cylinder. By controlling the air pressure discharged from the air bar, the amount of blanket wrap may be maintained within predetermined limits. In another embodiment, the air bar may be disposed proximate to the lower surface of the web, such that the air discharged from the bar acts in a direction to increase blanket follow. Thus blanket follow may be precisely controlled and variations in blanket follow minimized.
U.S. Pat. No. 7,627,141 to Noffke et al., entitled “System and method for measuring color on a printing press” is directed to an image processing apparatus and method for measuring spatial and spectral information from an image of a printed substrate. In one embodiment the apparatus processes spatial and spectral information from the same acquired image using first and second processors. In another embodiment, a first acquired image may be used to process the spatial information and a second acquired image may be used to process the spectral information. The first and second acquired images may be acquired at separate times during the travel of the printed substrate, both images being acquired at same point on the printed substrate. The color of the printed substrate can be monitored based on the processed spatial and spectral information.
U.S. Pat. No. 7,892,399 to Graham et al., entitled “Local tension generating air stabilization system for web products” is directed to an air stabilization system employing two parallel, opposite-facing Coanda nozzles. Each nozzle exhausts gas at opposite directions and subjects a moving flexible web to opposing forces effective to create local tension within the web. Each nozzle includes an elongated slot that is perpendicular to the path of the moving web. The nozzles serve as different points along the machine direction for controlling the height of the web. The operative surface with the nozzles can exhibit a flush surface. The nozzles can be formed on elevated structures on the operative surface. The operative surface can be covered with a transparent substrate to minimize shape distortions on the moving web and to prevent debris from collecting around a sensor. By modulating the velocities of gases exiting the nozzles, the shape of the web can be manipulated to present a planar contour. The air stabilization system can be incorporated into a caliper scanner.
U.S. Pat. No. 6,281,679 to King et al., entitled “Web thickness measurement system” is directed to a non-contacting thickness measurement apparatus which is capable of on-line web thickness measurements. The apparatus is comprised of a first and second distance determining means and an x-y position adjustment means. The first and second distance determining means calculate a distance value from each distance determining means to a web surface. The distance determining means are on opposite sides of the web, so each distance determining means measures the distance to a different side of the web. At the same time, the distance between the two distance determining means is measured to produce a z-sensor spacing value. The two distance values are thereafter subtracted from the z-sensor spacing value to get a web thickness value. The x-y position adjustment means is used to keep the measurement spot on the web for the two distance determining means at the same location on the web.
U.S. Pat. No. 6,382,100 to Satoh et al., entitled “Web guide roller, and printing press incorporating the same” is directed to a roller assembly for pneumatically guiding a web of paper after it has been printed upon without the risk of smearing. The roller assembly is positioned downstream of a printing station in a web-fed rotary printing press. The assembly includes a roller in the form of a hollow cylinder defining a plenum chamber and having formed on its surface a multiplicity of air outlet openings in communication with the plenum chamber. Air supplied under pressure to the chamber is thus expelled from the air outlet openings in order to avoid contact between the roller and the web traveling over the roller. A baffle encloses at least part of the circumferential part of the roller surface for confining air expelled from the other circumferential part of the roller surface and for redirecting the confined air into a pair of spaces spaced between the web and the roller.
U.S. Pat. No. 6,135,026 to Kalbantner et al., entitled “Sheet guiding device in a sheet-fed printing press” is directed to a sheet guiding device assemblable from at least a first sheet guiding element and a second sheet guiding element. The sheet guiding device is for guiding sheets of printing material in a sheet-fed printing press and includes a guide for guiding at least one of the sheet guiding elements as the sheet guiding elements are being mutually assembled. In one embodiment of the sheet guiding device, at least one of the sheet guiding elements is embodied as a sheet guiding baffle provided with nozzles with the sheet guiding device including a radial fan for supplying the nozzles with blown air.